Electronic devices without power adapters provide cost savings and a user friendly experience. One way to provide electronic devices without power adapters is by tapping Universal Serial Bus (USB) power. Some electronic devices tap USB power by simply connecting to a computer's USB host port without consideration for the USB host port's power management requirements. The lack of consideration for the USB host port's power management requirements can lead to a number of problems.
First, the USB host port may provide insufficient power for the electronic device to operate. Typical USB host ports supply current up to 100 mA in compliance with USB standard 7.2.1.3. if a proper initialization process is not performed. Further, if the USB host port implements over current protection and the electronic device requires more than 100 mA of current (i.e. high power) to operate, it is possible that the USB voltage (VBUS) will be turned off in response to connecting the electronic device to the USB host port. In some cases, even after the electronic device is disconnected from the USB host port, the USB host port may still be disabled and in an unusable state until the user manually resets the USB host port.
In addition to the possibility of the USB host port providing insufficient power, the electronic device will loose power when the USB host port enters a suspend mode. In compliance with the USB standard, the USB host port will suspend the USB bus if there are no data transactions over the USB bus for a certain time. After the USB host port suspends the USB bus for lack of data transactions, the USB power draw is limited to 500 μA and the electronic device may loose power or be reset.
Further, in addition to the possibility of the USB host port providing insufficient power or entering a suspend mode, tapping USB power without proper initialization may also damage the USB host port. In response to connecting an electronic device to a USB host port, the computer may implement an unrecoverable protection mechanism on the USB host port. Connecting an electronic device to the USB host port may generate inrush current. Depending on the protected current limitation of the USB host port, this inrush current may exceed the current limitation of the USB host port and result in an unrecoverable state where the USB host port is inoperable.
For these and other reasons there is a need for the present invention.